Downhole reservoir surveillance systems often consist of sensors (pressure & temperature) that are lowered into a well and cemented in place at specific elevations to make contact with the geological formation of interest, for the sake of measuring in-situ pressure and temperature. Often, these sensors are packaged in steel housings that are usually welded to the outside of the casing, and designed for mechanical protection of the delicate sensor. This way, the sensor is carried downhole with the casing that it is attached thereto. A signal cable runs from the sensor (downhole) to the surface, to convey the sensor measurements.
In many installations, more than one sensor is lowered into the same well, with each designed to measure physical phenomena within a zone of interest. In these types of installations, prevention of hydraulic communication between two or more zones of interest is preferable for measurement precision. The sensors are thus typically cemented in place within the wellbore, and it is the cement that acts as a barrier for migration of in-situ fluids from zone to zone. In this case, the potential for leakage in between zones however still remains due to the micro annulus formation around the sensor and the sensor signal cables from lower zones passing through upper zones of interest (since they run all the way up to surface). Therefore, if not cemented properly, the surrounding environment of the sensor(s) and the cables connected thereto may create a micro-annulus in which gas or liquid is able to travel, thus compromising the zonal isolation.
There is thus a need for a downhole surveillance system and method with improved downhole sensing apparatus which addresses at least some of the above noted limitations association with the prior art.